Hydraulic cylinders, such as are useful in numerous applications, have been provided with valved pistons, for use as shock absorbers, and with pistons including valves suiting them for shock absorber use on boats, for tilting, or for trimming and tilting a drive unit for a boat such as an outboard motor or an outdrive unit, for allowing it to pivot upwardly when the propulsion unit strikes an obstacle, to avoid damage to the propulsion unit, and to let the propulsion unit return to operative position in a controlled manner following contact with the obstacle, if warranted. Such shock absorbers have been provided with stop pistons, providing an adjustable retracted-position cylinder end, which could be moved under hydraulic control to set a trim position for the outdrive or outboard unit, and cause the piston, and the drive unit, to return to a preset trim position following collision with an obstacle.
In order to obtain the flow rates necessary through the shock absorber piston to allow rapid movement when an associated outboard motor or outdrive unit strikes an obstacle, numerous small valves are conventionally used. In one such system, a piston is provided with seven small valves, acting as paralleled jounce valves, for allowing the piston to move upon contact of the drive unit with an obstacle, and an eighth rebound valve to allow the drive unit to return to operative position. The use of this multitude of valves, while functional, has certain deficiences. For instance, the use of eight separate valves provides eight separate hydraulic leak paths, any of which may interfere with the proper operation of the hydraulic cylinder. Eight separate valve seats must be machined in bores through the piston, seven substantially identical precision springs must be selected or obtained, since it is not practicable to make all eight valves individually adjustable, or to adjust the valves to begin to open at a definite pressure in a simple manner. Also, the individual poppet-type relief valves are not precisely controllable, to provide an operating pressure which may be set to correspond to the mechanical strength of the outboard motor or outdrive unit members to act only when it is necessary to protect them from damage. If the pressure of these multiple relief valves is set too high, the structure of the drive unit will be damaged. If set too low, the drive unit may spend excessive time with its propeller out of the water, increasing the opportunity for engine damage, or, accelerating with the propeller rotation reversed may pull the outboard motor or outdrive unit out of the water, also causing overspeed. This adjustment is difficult and troublesome, since poppet-type valves are proportionally responsive, allowing some flow at a low pressure and greater flow at greater pressure. Valves which allow adequate flow at impact-caused pressure may allow the drive unit to gradually be pulled out of the water by reversed operation of the drive unit.
As a partial solution to this problem, a single large valve which operates in a stepwise manner at a predetermined and easily adjustable pressure has been provided, and placed adjacent to the hydraulic cylinder, to provide a precisely controllable operating pressure. Such a device is disclosed in co-pending application Ser. No. 146,891, filed May 5, 1980. This device, while providing improved performance, is placed remote from the piston of the hydraulic cylinder, so that its actuation in the presence of the impulse pressure caused by an outboard motor or outdrive unit striking an obstacle is delayed by the mechanical expansion of the components subject to the impulse pressure, so that the structure of the outboard motor or outdrive unit may be subjected to considerable mechanical stress before the pressure at the relief valve rises to its preset operating pressure, necessitating that the relief valve be set at a lower than optimum pressure to ensure protection of other mechanical components.
Also, in order to supply hydraulic fluid to both ends of a hydraulic cylinder, so that a piston, or a stop piston, may be moved in both directions within the cylinder, complicated and potentially fragile structure has been provided. Separate lines have been provided between a source of hydraulic fluid, such as a reversible hydraulic pump, and the opposite ends of the hydraulic cylinder. These separate lines provide two separate potential failure items, which may be degraded by age and weathering, or be caught and torn upon an obstacle. As is known, one end of a cylinder may be supplied with hydraulic fluid through a mounting pin, which includes conduits or passages which align with conduits or passages in the housing of the hydraulic cylinder. The opposite end of the hydraulic cylinder is conventionally supplied with fluid by placing a separate conduit or hose along the side of the housing of the hydraulic cylinder, and which may be fastened to the side of the hydraulic cylinder, such as by welding. It is also known to use a hollowed rib, such as a hollowed rib used to protect tie bolts used in a cylinder assembly with a separate cylinder tube and end caps, to convey working fluid to the end of the cylinder remote from the mounting pin or pivot.
Such external conduit means are often complex and difficult to assemble, contain numerous fittings which may be apt to leak, or present undesirable protrusions from the hydraulic cylinder.
The instant invention provides a hydraulic cylinder which overcomes these and other deficiencies of previous similar cylinders.